This invention relates to an iron alloy article formed by compacting and sintering a predominantly iron powder mixture that comprises carbon powder and a boron-containing additive. More particularly, this invention relates to a sintering aid added to the powder mixture to promote carbon diffusion, particularly within interior regions of a large compact, and thereby produce a more uniform matrix microstructure.
U.S. Pat. No. 4,618,473, issued to Jandeska in 1986, describes an iron alloy article produced by compacting and sintering a powder mixture composed predominantly of iron powder and containing a carbon powder and a nickel boron powder, preferably of intermetallic nickel boride compound. During sintering, the iron is diffusion bonded into an integral structure. Carbon diffuses into the iron to form a mainly pearlitic or martensitic product microstructure. Nickel and boron also diffuse into the iron, but nickel diffusion is localized in pore regions to form, upon cooling, retained austenite phase that enhances product toughness. Preferably, powdered copper is added for increased hardness and dimensional control.
It has also been found that, at suitable concentrations, boron that diffuses into the iron combines with carbon to produce dispersed, hard borocementite particles that improve wear resistance. U.S. Pat. No. 4,678,510, issued to Jandeska in 1987, describes sintering a predominantly iron powder compact containing carbon powder and boron-containing additive to produce the desired hard particles. The boron additive preferably includes both nickel boride powder and iron boride powder. In addition to forming the borocementite particles, carbon is also required to produce the desired martensitic or pearlitic matrix.
In the methods described in both patents, sintering is preferably carried out in a vacuum to eliminate oxygen that would otherwise react with boron. Boron oxide compound does not suitably relinquish boron to the iron in the desired manner.
In sintering iron powder articles having large cross sections, it has been found that sintering times adequate to bond the iron into a cohesive structure produce a desired martensitic or pearlitic microstructure in exterior regions, but that interior regions contain undiffused carbon particles and carbide-free ferrite grains. Ferrite is relatively soft and reduces product strength. We have found that the desired matrix microstructure may be formed in interior regions by extending the sintering time, for example, by up to a factor of 10, but at a substantial cost penalty. Since more uniform carburization is found in comparable compacts that do not include the metal boride additive, this delayed interior carburization is believed attributable to the presence of boron.
Therefore, it is an object of this invention to provide an improved method for forming a powder iron article comprising carbon and a boron-containing additive, which promotes carbon diffusion within interior compact regions during sintering that is comparable to carbon diffusion within exterior regions, despite the presence of boron, to produce a more uniform microstructure throughout the compact without a required extension of the sintering time.
More particularly, it is an object of this invention to provide an improved method for compacting and sintering a predominantly iron powder mixture comprising carbon powder and boron-containing additive, which method includes addition of a sintering aid to the powder mixture to promote carbon diffusion within interior regions of the compact and thereby to produce a more uniform matrix microstructure composed predominantly of martensite or pearlite. The sintering aid also promotes boron diffusion and in one aspect of this invention enhances formation of hard borocementite particles dispersed throughout the product, including within both interior and exterior regions.